This invention relates generally to the control of a high pressure fluid and, more particularly, to valves used to achieve pressure drops in a fluid flow line.
It is known in the art that several problems, including objectionable noise, are generated by the increase in velocity that follows from the reduction in pressure as expanding pressurized fluid flows through the orifice in a control valve. Put differently, fluctuations in flow through the orifice have a marked influence on the production, frequency and intensity of noise. Attempted solutions have included various devices for subdividing and confining the fluid in its flow through the orifice. Such devices are referred to as low noise trim and usually require an increase in the size of the valve for a given installation. For example, in U.S. Pat. No. 3,513,864 to Self, a stack of skeletal and imperforate discs is positioned as a cage around the valve plug. With such a cage, the number and length of tortuous paths required to achieve a desired pressure drop dictate a very large surface area and, therefore, a valve that is disproportionately large for the associated pipes. Another device, in the form of sandwiched discs and screens surrounding a conduit for a valving piston, is shown schematically in FIG. 3 of U.S. Pat. No. 3,802,537 to White. In such a device, flow through perforations in the conduit would create turbulence and noise. Restricted flow through the perforations would also create back pressure, thus achieving a less effective flow across the screens.